TPS23754: RE failure between 30MHz to 40Mhz

Hi Aswin,

Removing D6 and R11 can increase the switching off time, which normally results slightly increased switching loss (the major switching loss comes from switching on normally). 

The value of R13 normally depends on the input capacitor of the Q1, you can try 5 - 30 Ohm normally.

Sometimes adjusting the snubber circuits of Q1 and D35 can help with reducing 30MHz to 40MHz noise amplitude. 

Best regards,

Diang

Hi Diang,

Thanks for your reply, We have a few more concerns on this.

Is it ok to use a 100E resistor for R13, as we are getting better results with 100E than lower values like 30E, Is there any issue in taking this approach?
attaching the waveform at mosfet gate when using 22E as R13 (fig 1) and using 100E as R13 (fig 2).

Fig 1. R13 = 22E

Fig 2. R13 =100E

Hi Aswin,

Thanks for your reply. Yes R13 = 100Ohm shows much clean Vgs waveform. 

May I know the part# of Q1? If Q1 is a very low current rated device, 50 - 100 Ohm gate resistor may be quite suitable.

Best regards,

Diang

Hi Diang,

 We are using the part SI7898DP-TI-GE3 FROM Vishay for Q1

Hi Aswin,

Thanks for your information.

Looks like SI7898DP-TI-GE3 has 900 pF input capacitance, which is a very common value for PoE in that power rating. I would suspect the Vgs oscillation could be caused by the coupling of Vds during turning on.

While since there is no over voltage stress on Vgs, and if there is no thermal issue and Vds over volatge stress, there is no problem to have 100-Ohm gate resistor.

Best regards,

Diang

Hi Aswin,

I will close this thread for now. Please reply or open a new thread if you have further questions.

Best regards,

Diang